Storage devices using semiconductor elements are broadly classified into two categories: a volatile device that loses stored data when power supply stops, and a non-volatile device that holds stored data even when power is not supplied.
A typical example of a volatile storage device is a dynamic random access memory (DRAM). A DRAM stores data in such a manner that a transistor included in a storage element is selected and charge is accumulated in a capacitor.
When data is read from a DRAM, charge in a capacitor is lost on the above-described principle; thus, another writing operation is necessary whenever data is read. A data holding period is short because charge flows from/into a transistor forming a memory element by a leakage current between a source and a drain in an off state (off-state current) or the like even when the transistor is not selected. For that reason, another writing operation (refresh operation) is necessary at predetermined intervals, and it is difficult to sufficiently reduce power consumption. Further, since stored data is lost when power supply stops, an additional storage device using a magnetic material or an optical material is needed in order to hold the data for a long time.
Another example of a volatile storage device is a static random access memory (SRAM). An SRAM holds stored data by using a circuit such as a flip-flop and thus does not need refresh operation, which is an advantage over a DRAM. However, cost per storage capacity is increased because a circuit such as a flip-flop is used. Moreover, as in a DRAM, stored data in an SRAM is lost when power supply stops.
A typical example of a non-volatile storage device is a flash memory. A flash memory includes a floating gate between a gate electrode and a channel formation region in a transistor and stores data by holding charge in the floating gate. Therefore, a flash memory has advantages in that the data holding time is extremely long (almost permanent) and refresh operation which is necessary in a volatile storage device is not needed (e.g., see Patent Document 1).
However, a gate insulating layer included in a storage element deteriorates by tunneling current generated in writing, so that the storage element stops its function after a predetermined number of writing operations. In order to reduce adverse effects of this problem, a method in which the number of writing operations for storage elements is equalized is employed, for example; however, a complicated peripheral circuit is needed to realize this method. Even when such a method is employed, the fundamental problem of lifetime is not solved. In other words, a flash memory is not suitable for applications in which data is frequently rewritten.
In addition, high voltage is necessary for injecting charge in the floating gate or removing the charge, and a circuit for generating high voltage is also necessary. Further, it takes a relatively long time to inject or remove charge, and it is not easy to perform writing and erasing at higher speed.